Television picture images have two spatial dimensions, i.e. horizontal and vertical. Such images also have a time based or temporal dimension. The present discussion concerns the spatial dimensions. The aspect ratio of a television picture image is a ratio of horizontal length to vertical length, expressed in relative units. Conventional picture images have an aspect ratio of four to three, or 1.33 (when reduced). Much, but not all of, the programming material made for television follows the 1.33 aspect ratio, whether it is originated as video or cinema.
Aspect ratio is essentially an aesthetic consideration or value, i.e. what is the most pleasing rectangular shape to behold. The ancient Greek civilization developed a so-called Golden Number for rectangles in architecture. This Golden Number had an aspect ratio of about 1.62, and it is found today in the Greek ruins, such as the Parthenon in Athens. The Japanese national broadcasting service, NHK, made a study with its viewers to determine the most desirable aspect ratio for television picture viewing, incident with its development of standards for high definition television (HDTV). This study determined that 4.8 to 3 (i.e. about 1.62) was subjectively the most pleasing picture image arrangement for the human eye.
The apparent desirability of a 1.62 aspect ratio may be explained by reference to the human eye-brain mechanism and particularly by reference to the elliptical shape of the human retina. Irrespective of the precise explanation for subjective preference for a 1.62 aspect ratio for rectangles, a popular viewpoint has recently begun to surface that the conventional 1.33 television picture image aspect ratio looks too square. It looks like the aspect ratio found in old movies, and is therefore undesirably old fashioned.
With the advent of HDTV schemes have come proposals that wider aspect ratios are more aesthetically pleasing than the conventional ratio, and should be adopted. Ratios of five to three (1.66) and sixteen to nine (1.77) have frequently been proposed. Also, modern cinematographic theater movies, not made expressly for conventional television, are typically filmed with aspect ratios greater than 1.33. These theater films typically have aspect ratios which vary from about 1.65 to 2.35, the 2.35 ratio being associated with anamorphic Cinemascope (tm), or multiple film, very wide display theatrical movies.
A commonly encountered aspect ratio for modern theater movies is 1.85. These theater movies have been modified for presentation on conventional television, either by "pan and scan" techniques, or by optical compression resulting in size distortion in the resultant video picture images. The emerging preference among HDTV schemes for a 1.77 aspect ratio is said to be justified by the modern movie aspect ratio of 1.85.
In order to accomodate the increasing quantities of worthwhile programming material having disparate aspect ratios, it is very desirable for a television system to have a variable aspect ratio capability. However, this capability is not found in present television receivers following the conventional aspect ratio, and because of the great numbers of conventional sets, it is appropriate and necessary for any variable aspect ratio proposal to maintain downward compatibility with the present NTSC color signal format.
It has also been proven to be desirable to provide consumers with ACTV or HDTV receivers, which not only display a larger number of scanning lines (e.g. 1050 lines) but which are also compatible with the present NTSC color television signal format standard and are also making use of a wide display screen having, for example, an aspect ratio of 1.77.
A hitherto unsolved need has arisen for a television system which provides a selectable or variable aspect ratio, while maintaining full compatibility with the present NTSC standard. Such a television system should be capable of displaying undistorted images on conventional receiver sets having narrow aspect ratio screens (1.33), possibly with black bands appearing at the top and bottom of the picture display area in the case of wide aspect ratio picture signal transmissions, while displaying full screen, wide aspect ratio pictures on new sets using wide aspect ratio display screens without black bands at the top or bottom of the picture.